Sunday, May 13, 2001		Nature

These insects lure the prey with light
By Nutan Shukla

ANIMALS attract prey in different ways. One of the most interesting case is that of the larvae of a small fly found in New Zealand. In the Waitomo Caves of the country, these luminous insects have become a tourist attraction because their lures are one of the great wonders of nature.

Protected by a silken sheath, these larvae cling to the ceiling of the cave and exude sticky threads, each having a number of globules of a mucous-like substance. From six inches to two-feet-long, these necklace-like structures have globules at two inch intervals appearing like pearls. Each individual creates from 20 to 70 such strands. Hanging from the roof of the dark caves, hundreds of these glowing strands form a bright, luminescent, bluish-green curtain which is visible even in the dark. Now the question is how these pearl-strings glow in total darkness? The answer is, the luminous organ is in the larva’s tail, which shines down on the mucous, reflecting on the strands, making the whole structure visible in the darkness. Prey insects, attracted by the light, are caught and trapped on the strands and are reeled in like fish on fishing lines.

Luminosity of the glow-worm is primarily an aid to courtship. For one particular species it is also a way of attracting prey. The predatory female fireflies of one species mimic the luminous call-sign of another species. The males of the prey species are attracted by what they believe to be a female of their own species, but quickly discover that they have been enticed to their death.

The deep-sea, scaled dragon fish have luminous chin barbels that are thought to attract prey to the mouth. One species sports a barbel 10 times the length of its body with a tip that is luminous.

The black angler fish, which lives at about 1,200 ft at the bottom of the sea, has a luminous forked structure on the roof of its mouth. One species of hatchet fish probably attracts prey right into its mouth with the aid of two patches of luminescent tissue that can glow for 30 minutes at a time. One species of lantern fish has light organs on its tongue.

What causes luminescence of the organs in these creatures? Light is not produced by the fish but by symbiotic bacteria that are cultured in special compartments within the lure. They are encouraged to glow when oxygenated blood is pumped into the chamber. When at rest, the fish shuts down its light organs simply by temporarily cutting off the blood supply.

The mid-water viper fish behave in a way similar to the angler fish. They have a modified second dorsal fin spine, tipped with a luminous lure, that has become even more elongated. The predator hangs motionless in the water, its head lower than its tail, with the fin-ray reaching over the head so that the lure dangles in front of the mouth. They look fearsome, with enormous mouths and gigantic teeth, but, fortunately, they are only two to 12 inches long.

Siphonophores are free-floating, colonial relatives of the jellyfish. They might be considered as ‘simple’ animals, yet these tiny creatures show an amazingly sophisticated method of luring prey to their tentacles. Some species of siphonophores are capable of movement and can wriggle themselves into the centre of dense patches of prey where they take pot-luck. Other slow-moving relatives allow nothing to chance.

In one type, the sting cells with which they kill their prey are grouped together into red-coloured batteries of cells that have two sensitive hairs that trigger all the cells in the group. The battery of cells mimics the copepod (tiny crustaceans that live in the zooplanktons) prey that they catch even to the point of pulsing like copepods. Another species has the sting cells shaped like tiny fish larvae, complete with eyes and fins. They trap the small sea creatures that prey on fish larvae using the battery of mimicking cells to entice them closer.